Electron Gun + Polywell Results

Today, we tested the electron gun in tandem with the Polywell core. Right now, our goal is to simply understand effect the e-gun has on the potential well, if it has any effect at all.

We had the pump running since 3AM the night before, and so by about 2PM today, we had a vacuum in the 5 x 10^-5 range.

Not super amazing, but certainly good enough for out purposes.

First, we mounted the power supplies in the rack.

Hooked them up to their proper feed-through pins.

Attached the langmuir probe and the shunt resistor to to the oscilloscope, allowing us to monitor the potential well depth and current going through the coils simultaneously and in real time.

Then we began testing. The coils in the core worked great.

The spike on the top line is the power supply’s capacitor bank discharging. The smaller spike on the lower line is a current induced in the langmuir probe by the sudden appearence and dissappearence of a magnetic field generated by the coils in the core. So the Polywell works.

Then we tried the electron gun, and it didn’t work at all. the filament was glowing, and we were getting high voltage on the accelerator’s feedthrough pins, but no reading on the langmuir probe.

After extensive thinking, speculating, and white-board writing, we decided we had to open the chamber up to see if the connections to the accelerator were right.

Here’s what we found

That’s your problem right there, ma’am

It’s a little hard to see, but a gray plastic piece which connects the core to the feed-through pins was right in front of the accelerator, totally blocking the beam.

Funny how sometimes the causes of problems are so obvious that you don’t even think of them.

Anyway, I got that fixed, tried it again, and got a beam. The readings on the langmuir probe attached to the multimeter were more or less the same as those from the last test. When we attached it to the oscilloscope however, things were a little more complicated.

The beam intensity was not static, but periodic. It fluctuated with a frequency of 60 Hz, pointing to the AC current which powers the hot cathode.

We expected something like this, because the availability of electrons to accelerate fluctuates with the AC powering the hot cathode.

Ideally, our electron beam would be have a perfectly even intensity, because then we could eliminate it as a variable.

Fixing that would involve rectifying the AC, a major upgrade to the power supply, so we decided to leave that for another day and run the experiment.

Here are some of the results

A really good one. The downward spike on the lower line signifies a a potential well. Nice!

Here we see a well, but it’s at the wrong time, it seems to have appeared just after we pulsed the core.

Here’s a strange one in which the Polywell core pulse seems to cause some change in the voltage on the langmuir, but not a well.

This one really demonstrates why the periodic electron beam is such a problem. The top spike came at a moment when the langmuir probe was reading zero. This means that there was no beam when we pulsed the core, so it’s no surprise that we just got an induced current.

The most tantalizing and baffling run we did. The well appears to be extremely deep, greater than 100V, but it’s unclear whether thats credible, because there’s so much other confusing stuff going on.

All in all, our results are little confusing, but good. We were able to create the well, which is a big win, but we weren’t able to do so consistently. In order to really study how the well is affected by tweaking variables, we need consistent baseline well to compare against.

Good thinking, if we pulsed the core that frequently, the coils would melt. We have to give it a little bit of time between each shot to cool, but we might try less frequent synchronized pulsing, one core pulse every 6000 cycles of the electron gun, perhaps. We’ll definitely think about that.

Maybe it would be worth adding a photodetector inside the vacuum chamber to look for flashes of light coincident with arcing… BW always said there was maximum signal from the photodiode just as the PW went into WB mode….

Maybe leaving the 60 Hz in there was a stroke of serendipity! You would have been scanning through different electron beam energies and currents as you were also scanning through different magnetic field strengths…

Sometimes with experimental work, running with something you *know* is “slightly broken” turns out not to be so broken after all…

I’d consider trying to synchronize your cap bank discharge with the 60 Hz mains as Wayne Farmer suggests, prior to trying to suppress all the AC on the heater. Use a diac+triac with an adjustable phase delay – look up “dimmer switch” circuits. You can probably just use a little transformer to couple the triac’s pulse out to trip your bank. That circuit will let you adjust the “phase” relatively easily as well.

Another thing you could do is, after changing the filament over to DC heating, split the accelerator anode so you have a “gate” ring closer to the cathode. You could then use this with a relatively small potential to control the electron beam current – with the accelerator potential to control the electron beam energy.

But before you change anything, just repeat – do another run with the same settings, and get some more traces recorded, maybe that last shot will just happen every now and then?

Might be worth automating the cap discharge, so you can just leave it running overnight – don’t bother synchronizing it, just let it slowly perform the parametric sweeps for you. :) Good way to collect a lot of data to look at later.

The fact that you can see the filament AC suggests your accelerating electrode is zero volts. Valves have used 6.3v AC for years but the filament voltage polarity has little effect on the electron current unless the plate voltage is very low (like a valve diode)
Dustin

On second thought, it may be because your magrid is at zero volts, the electrons are accelerated by your focus electrode but repelled or slow down as they aproach the magrid because it is almost at the same potential as the filament. You either have to float the filament AC drive at a negative DC voltage or your Capacitor discharge bank positive with respect to the filament so the electrons have enough energy to enter the magrid. Its easier to think of this as a valley with the filament at the top on one side, the focus electrode at the bottom of the valley and the magrid at the top on the other side. The electrons will roll down and up the other side and roll back again. You only see the negative pulses as the filament AC gives it just enough energy on half the cycle to enter the magrid. You have to either raise the filament or lower the magrid. Changing the focus voltage will not help.
Dustin

The way the magrid of a polywell is usually depicted in Bussard’s descriptions has the coil containers as the “accelerating anode”. Ideally the coils and their power supply would all float at full electron drive positive potential.

For small electron drive voltage (not many kV) this could be done by having insulation between the coils and the magrid coil casings (which are not attached, here). That way the coils could actually be at any convenient voltage, whilst contained in well insulated containers at anode potential.

This trick becomes difficult for full scale, as you run out of space for the insulation at full voltage within the coil containers. Any insulation failure there blows up the machine, as the electron short circuits to the coil supply. Easier in this case to just float the whole coil current supply at full potential, and then you needn’t worry about much insulation between the coils and the containers.

For this rig, I’d try putting a loose “anode grid cage” about each of the coils – could just be a loose winding about each toroid, and have this used as the accelerating anode as one connected piece. For decent vacuum and relatively short distance, it should hold off a fairly large voltage – these could be looped about the outside of the teflon coil formers, and won’t need much support.

Firing the coil supply shouldn’t induce much current in the anode – and it wouldn’t matter much if it did – each winding need not actually form a circuit – it’s only job is to act as a cage that can have positive voltage connected to it, without shorting to the magrid coils themselves.

Of course, rather than having a large positive voltage on the guts of the thing, and the container + guns grounded, one could instead have the guts at ground, and have a large negative voltage on the container + guns. This does tend to mean a lot of arcing from the vacuum chamber and pump to everything nearby, and so I don’t think it’s a particularly good idea – much better to have to deal with a HV+ cage with a coil supply in it (or a very well insulated transformer to drive the coil supply, or even a long insulated rod with a motor at one end and a generator at another…) than have the whole apparatus at deadly voltage. Either way there will be a need for high voltage standoffs through the vacuum chamber, and the effective electric field within the device itself is identical (as it has to be).